[Devel] [PATCH RH9 18/30] modules: use kvmalloc when creating sysfs attributes for ELF sections

[Andrey Zhadchenko]

From: Evgenii Shatokhin <eshatokhin at virtuozzo.com>

A kernel module containing a ReadyKernel patch could have lots of ELF
sections: one per each new or patched function, one per each new static
or global variable, etc.

The kernel creates a sysfs file
/sys/module/<module_name>/sections/<section_name> for each loaded
section when the patch module is being loaded, see add_sect_attrs() in
kernel/module.c. A big chunk of memory is allocated for all these with
kzalloc.

For the ReadyKernel patches we have already released, the amount of
memory could be as high as 34528 bytes (48 + 80 * 431 loaded sections),
which is a 4th order allocation. 3rd order allocations are also common
here, see https://jira.sw.ru/browse/PSBM-95050.

Not only it is a waste (contiguous memory is not needed there), but the
allocation may also fail when the memory is fragmented. ReadyKernel
patches are often used in the systems with rather significant uptime, so
the fragmentation is possible.

It could be better if the patch modules did not use too many ELF
sections. However, the KPatch maintainers pointed out
(https://github.com/dynup/kpatch/pull/1131) that the same problem would
affect regular kernel modules as well after FGKASLR has been merged into
the mainline kernel. Combining the sections of the kernel modules
destroys the purpose of FGKASLR, so, it was agreed that we probably
should just switch to kvmalloc+kvfree in add_sect_attrs/free_sect_attrs.

Details and discussion:
https://www.spinics.net/lists/live-patching/msg06364.html

https://jira.sw.ru/browse/PSBM-108017
Signed-off-by: Evgenii Shatokhin <eshatokhin at virtuozzo.com>

(cherry picked from vz8 commit e16aab11613f1fcf9b34d3df664591e5504fa624)
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
---
 kernel/module.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index c402a69..05cb510 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1529,7 +1529,7 @@ static void free_sect_attrs(struct module_sect_attrs *sect_attrs)
 
 	for (section = 0; section < sect_attrs->nsections; section++)
 		kfree(sect_attrs->attrs[section].battr.attr.name);
-	kfree(sect_attrs);
+	kvfree(sect_attrs);
 }
 
 static void add_sect_attrs(struct module *mod, const struct load_info *info)
@@ -1546,7 +1546,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info)
 	size[0] = ALIGN(struct_size(sect_attrs, attrs, nloaded),
 			sizeof(sect_attrs->grp.bin_attrs[0]));
 	size[1] = (nloaded + 1) * sizeof(sect_attrs->grp.bin_attrs[0]);
-	sect_attrs = kzalloc(size[0] + size[1], GFP_KERNEL);
+	sect_attrs = kvzalloc(size[0] + size[1], GFP_KERNEL);
 	if (sect_attrs == NULL)
 		return;
 
-- 
1.8.3.1